I believe that Hearns poor infighting ability comes from his excelent jab, goodnight right and his all around boxing kills
His timing was solid so he could land very effective against someone coming in. Hagler was a monster, the irressistible force and made good use of his chin and musclepower.
Against Floyd Tommy has the skills to fire the jab at will, keeping Floyd on the outside and wait patiently for the opening for the right